Position location using digital video broadcast television signals

ABSTRACT

Apparatus to determine the position of a user terminal, the apparatus having corresponding methods and computer-readable media, comprises a receiver to receive, at the user terminal, a wireless orthogonal frequency-division multiplexing (OFDM) signal comprising a scattered pilot signal; and a processor to determine a pseudo-range based on the scattered pilot signal; wherein a position of the user terminal is determined based on the pseudo-range and a location of a transmitter of the OFDM signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a CONT of 09/932,010 Aug. 17, 2001, now U.S. Pat.No. 7,126,536

which is a CIP of Ser. No. 09/887,158 Jun. 21, 2001 ABN

and which claims the benefit of 60/265,675 Feb. 02, 2001

and which claims the benefit of 60/281,270 Apr. 03, 2001

and which claims the benefit of 60/281,269 Apr. 03, 2001

and which claims the benefit of 60/293,812 May 25, 2001

and which claims the benefit of 60/293,813 May 25, 2001

and which claims the benefit of 60/293,646 May 25, 2001.

This application is related to 60/737,027 Nov. 14, 2005.

The subject matter of all of the foregoing are incorporated herein byreference.

BACKGROUND

The present invention relates generally to position determination, andparticularly to position determination using DTV signals.

There have long been methods of two-dimensional latitude/longitudeposition location systems using radio signals. In wide usage have beenterrestrial systems such as Loran C and Omega, and a satellite-basedsystem known as Transit. Another satellite-based system enjoyingincreased popularity is the Global Positioning System (GPS).

Initially devised in 1974, GPS is widely used for position location,navigation, survey, and time transfer. The GPS system is based on aconstellation of 24 on-orbit satellites in sub-synchronous 12 hourorbits. Each satellite carries a precision clock and transmits apseudo-noise signal, which can be precisely tracked to determinepseudo-range. By tracking 4 or more satellites, one can determineprecise position in three dimensions in real time, world-wide. Moredetails are provided in B. W. Parkinson and J. J. Spilker, Jr., GlobalPositioning System-Theory and Applications, Volumes I and II, AIAA,Washington, D.C. 1996.

GPS has revolutionized the technology of navigation and positionlocation. However in some situations, GPS is less effective. Because theGPS signals are transmitted at relatively low power levels (less than100 watts) and over great distances, the received signal strength isrelatively weak (on the order of −160 dBw as received by anomni-directional antenna). Thus the signal is marginally useful or notuseful at all in the presence of blockage or inside a building.

There has even been a proposed system using conventional analog NationalTelevision System Committee (NTSC) television signals to determineposition. This proposal is found in a U.S. patent entitled “LocationDetermination System And Method Using Television Broadcast Signals,”U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the presentanalog TV signal contains horizontal and vertical synchronization pulsesintended for relatively crude synchronization of the TV set sweepcircuitry. Further, in 2006 the Federal Communication Commission (FCC)will consider turning off NTSC transmitters and reassigning thatvaluable spectrum so that it can be auctioned for other purposes deemedmore valuable.

SUMMARY

In general, in one aspect, the invention features an apparatus todetermine the position of a user terminal, comprising: a receiver toreceive, at the user terminal, a wireless orthogonal frequency-divisionmultiplexing (OFDM) signal comprising a scattered pilot signal; and aprocessor to determine a pseudo-range based on the scattered pilotsignal; wherein a position of the user terminal is determined based onthe pseudo-range and a location of a transmitter of the OFDM signal.

In some embodiments, the OFDM signal comprises a Digital VideoBroadcasting (DVB) signal. In some embodiments, the DVB signal comprisesat least one of: a DVB-Terrestrial (DVB-T) signal; and a DVB-Handheld(DVB-H) signal. In some embodiments, the processor determines theposition of the user terminal based on the pseudo-range and the locationof the transmitter of the OFDM signal. In some embodiments, to determinethe position of the user terminal, the processor determines an offsetbetween a local time reference in the user terminal and a master timereference, and determines the position of the user terminal based on thewireless OFDM signal, the location of the transmitter of the OFDMsignal, and the offset. In some embodiments, the processor determines asubsequent position of the user terminal using the offset.

In general, in one aspect, the invention features an apparatus todetermine the position of a user terminal, comprising: receiver meansfor receiving, at the user terminal, a wireless orthogonalfrequency-division multiplexing (OFDM) signal comprising a scatteredpilot signal; and processor means for determining a pseudo-range basedon the scattered pilot signal; wherein a position of the user terminalis determined based on the pseudo-range and a location of a transmitterof the OFDM signal.

In some embodiments, the OFDM signal comprises: a Digital VideoBroadcasting (DVB) signal. In some embodiments, the DVB signal comprisesat least one of: a DVB-Terrestrial (DVB-T) signal; and a DVB-Handheld(DVB-H) signal. In some embodiments, the processor means determines theposition of the user terminal based on the pseudo-range and the locationof the transmitter of the OFDM signal. In some embodiments, to determinethe position of the user terminal, the processor means determines anoffset between a local time reference in the user terminal and a mastertime reference, and determines the position of the user terminal basedon the wireless OFDM signal, the location of the transmitter of the OFDMsignal, and the offset. In some embodiments, the processor meansdetermines a subsequent position of the user terminal using the offset.

In general, in one aspect, the invention features a method fordetermining the position of a user terminal, comprising: receiving, atthe user terminal, a wireless orthogonal frequency-division multiplexing(OFDM) signal comprising a scattered pilot signal; and determining apseudo-range based on the scattered pilot signal; wherein a position ofthe user terminal is determined based on the pseudo-range and a locationof a transmitter of the OFDM signal.

In some embodiments, the OFDM signal comprises: a Digital VideoBroadcasting (DVB) signal. In some embodiments, the DVB signal comprisesat least one of: a DVB-Terrestrial (DVB-T) signal; and a DVB-Handheld(DVB-H) signal. Some embodiments comprise determining an offset betweena local time reference in the user terminal and a master time reference;and determining the position of the user terminal based on the wirelessOFDM signal, the location of the transmitter of the OFDM signal, and theoffset. Some embodiments comprise determining a subsequent position ofthe user terminal using the offset.

In general, in one aspect, the invention features computer-readablemedia embodying instructions executable by a computer to perform amethod for determining the position of a user terminal, the methodcomprising: receiving, at the user terminal, a wireless orthogonalfrequency-division multiplexing (OFDM) signal comprising a scatteredpilot signal; and determining a pseudo-range based on the scatteredpilot signal; wherein a position of the user terminal is determinedbased on the pseudo-range and a location of a transmitter of the OFDMsignal.

In some embodiments, the OFDM signal comprises: a Digital VideoBroadcasting (DVB) signal. In some embodiments, the DVB signal comprisesat least one of: a DVB-Terrestrial (DVB-T) signal; and a DVB-Handheld(DVB-H) signal. In some embodiments, the method further comprises:determining an offset between a local time reference in the userterminal and a master time reference; and determining the position ofthe user terminal based on the wireless OFDM signal, the location of thetransmitter of the OFDM signal, and the offset. In some embodiments, themethod further comprises: determining a subsequent position of the userterminal using the offset.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts an implementation of the present invention including auser terminal that communicates over an air link with a base station.

FIG. 2 illustrates an operation of an implementation of the invention.

FIG. 3 depicts the geometry of a position determination using 3 DTVtransmitters.

FIG. 4 depicts an implementation of a receiver for use in generating apseudo-range measurement.

FIG. 5 illustrates a simple example of a position location calculationfor a user terminal receiving DTV signals from two separate DTVantennas.

FIG. 6 depicts the effects of a single hill on a circle of constantrange for a DTV transmitter that is located at the same altitude as thesurrounding land.

FIG. 7 shows the carrier numbers for the first 50 continuous pilotcarriers.

FIG. 8 depicts the first 50 carriers of the continuous pilot carriers.

FIG. 9 depicts the autocorrelation function of the composite continuouspilot carriers with 177 parallel carriers in the 8K mode.

FIG. 10 depicts the frequency hopping of the first 5 scattered pilotcarriers.

FIG. 11 depicts the waveform of one example carrier with no signreversals over 8 time increments.

FIG. 12 is another view of the scattered pilot carriers.

FIG. 13 depicts the autocorrelation function of the composite set of 568frequency-hopped scattered pilot carriers.

FIG. 14 shows the detailed fine structure of the scattered pilotcomposite signal observed over the first 100 time increments.

FIG. 15 shows the fine structure of the doublet sidelobe of thescattered pilot composite carrier.

FIG. 16 depicts an implementation of a monitor unit.

FIG. 17 illustrates one implementation for tracking in software.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Introduction

Digital television (DTV) is growing in popularity. DTV was firstimplemented in the United States in 1998. As of the end of 2000, 167stations were on the air broadcasting the DTV signal. As of Feb. 28,2001, approximately 1200 DTV construction permits had been acted on bythe FCC. According to the FCC's objective, all television transmissionwill soon be digital, and analog signals will be eliminated. Publicbroadcasting stations must be digital by May 1, 2002 in order to retaintheir licenses. Private stations must be digital by May 1, 2003. Over1600 DTV transmitters are expected in the United States.

Other regions are implementing similar DTV systems. The EuropeanTelecommunications Standards Institute (ETSI) has defined a terrestrialDTV signal for Europe, referred to herein as the Digital VideoBroadcasting-Terrestrial (DVB-T) signal. These new DTV signals permitmultiple standard definition TV signals or even high definition signalsto be transmitted in the assigned 8 MHz channel. These new DVB-TDTVsignals are completely different from the analog NTSC TV signals, aretransmitted on new 8 MHz frequency channels, and have completely newcapabilities. The inventors have recognized that the DVB-T signal can beused for position location, and have developed techniques for doing so.These techniques are usable in the vicinity of DVB-T DTV transmitterswith a range from the transmitter much wider than the typical DTVreception range. Because of the high power of the DTV signals, thesetechniques can even be used indoors by handheld receivers, and thusprovide a possible solution to the position location needs of theEnhanced 911 (E911) system.

The techniques disclosed herein can be applied to other DTV signals thatinclude known sequences of data by simply modifying the correlator toaccommodate the known sequence of data, as would be apparent to oneskilled in the relevant arts. These techniques can also be applied to arange of other orthogonal frequency-division multiplexing (OFDM) signalssuch as satellite radio signals.

In contrast to the digital pseudo-noise codes of GPS, the DTV signalsare received from transmitters only a few miles distant, and thetransmitters broadcast signals at levels up to the megawatt level. Inaddition the DTV antennas have significant antenna gain, on the order of14 dB. Thus there is often sufficient power to permit DTV signalreception inside buildings.

As described below, implementations of the present invention utilize acomponent of the DVB-T signal that is referred to as the “scatteredpilot signal.” The use of the scattered pilot signal is advantageous forseveral reasons. First, it permits position determination indoors, andat great distances from DTV transmitters. Conventional DTV receiversutilize only one data signal at a time, and so are limited in range fromthe DTV transmitter by the energy of a single signal. In contrast,implementations of the present invention utilize the energy of multiplescattered pilot signals simultaneously, thereby permitting operation atgreater range from DTV transmitters than conventional DTV receivers.Further, the scattered pilots are not modulated by data. This isadvantageous for two reasons. First, all of the power in the scatteredpilots is available for position determination; none of the power isdevoted to data. Second, the scattered pilots can be observed for longperiods of time without suffering the degradation that data modulationwould produce. Thus the ability to track signals indoors at substantialrange from the DTV tower is greatly expanded. Furthermore, through theuse of digital signal processing it is possible to implement these newtracking techniques in a single semiconductor chip.

Referring to FIG. 1, an example implementation 100 includes a userterminal 102 that communicates over an air link with a base station 104.In one implementation, user terminal 102 is a wireless telephone andbase station 104 is a wireless telephone base station. In oneimplementation, base station 104 is part of a mobile MAN (metropolitanarea network) or WAN (wide area network).

FIG. 1 is used to illustrate various aspects of the invention but theinvention is not limited to this implementation. For example, the phrase“user terminal” is meant to refer to any object capable of implementingthe DTV position location described. Examples of user terminals includePDAs, mobile phones, cars and other vehicles, and any object which couldinclude a chip or software implementing DTV position location. It is notintended to be limited to objects which are “terminals” or which areoperated by “users.”

Position Location Performed by a DTV Location Server

FIG. 2 illustrates an operation of implementation 100. User terminal 102receives DTV signals from a plurality of DTV transmitters 106A and 106Bthrough 106N (step 202).

Various methods can be used to select which DTV channels to use inposition location. In one implementation, a DTV location server 110tells user terminal 102 of the best DTV channels to monitor. In oneimplementation, user terminal 102 exchanges messages with DTV locationserver 110 by way of base station 104. In one implementation userterminal 102 selects DTV channels to monitor based on the identity ofbase station 104 and a stored table correlating base stations and DTVchannels. In another implementation, user terminal 102 can accept alocation input from the user that gives a general indication of thearea, such as the name of the nearest city; and uses this information toselect DTV channels for processing. In one implementation, user terminal102 scans available DTV channels to assemble a fingerprint of thelocation based on power levels of the available DTV channels. Userterminal 102 compares this fingerprint to a stored table that matchesknown fingerprints with known locations to select DTV channels forprocessing.

User terminal 102 determines a pseudo-range between the user terminal102 and each DTV transmitter 106 (step 204). Each pseudo-rangerepresents the time difference (or equivalent distance) between a timeof transmission from a transmitter 108 of a component of the DTVbroadcast signal and a time of reception at the user terminal 102 of thecomponent, as well as a clock offset at the user terminal.

User terminal 102 transmits the pseudo-ranges to DTV location server110. In one implementation, DTV location server 110 is implemented as ageneral-purpose computer executing software designed to perform theoperations described herein. In another implementation, DTV locationserver is implemented as an ASIC (application-specific integratedcircuit). In one implementation, DTV location server 110 is implementedwithin or near base station 104.

The DTV signals are also received by a plurality of monitor units 108Athrough 108N. Each monitor unit can be implemented as a small unitincluding a transceiver and processor, and can be mounted in aconvenient location such as a utility pole, DTV transmitters 106, orbase stations 104. In one implementation, monitor units are implementedon satellites.

Each monitor unit 108 measures, for each of the DTV transmitters 106from which it receives DTV signals, a time offset between the localclock of that DTV transmitter and a reference clock. In oneimplementation the reference clock is derived from GPS signals. The useof a reference clock permits the determination of the time offset foreach DTV transmitter 106 when multiple monitor units 108 are used, sinceeach monitor unit 108 can determine the time offset with respect to thereference clock. Thus, offsets in the local clocks of the monitor units108 do not affect these determinations.

In another implementation, no external time reference is needed.According to this implementation, a single monitor unit receives DTVsignals from all of the same DTV transmitters as does user terminal 102.In effect, the local clock of the single monitor unit functions as thetime reference.

In one implementation, each time offset is modeled as a fixed offset. Inanother implementation each time offset is modeled as a second orderpolynomial fit of the formOffset=a+b(t−T)+c(t−T)²  (1)that can be described by a, b, c, and T. In either implementation, eachmeasured time offset is transmitted periodically to the DTV locationserver using the Internet, a secured modem connection or the like. Inone implementation, the location of each monitor unit 108 is determinedusing GPS receivers.

DTV location server 110 receives information describing the phase center(i.e., the location) of each DTV transmitter 106 from a database 112. Inone implementation, the phase center of each DTV transmitter 106 ismeasured by using monitor units 108 at different locations to measurethe phase center directly. In another implementation, the phase centerof each DTV transmitter 106 is measured by surveying the antenna phasecenter.

In one implementation, DTV location server 110 receives weatherinformation describing the air temperature, atmospheric pressure, andhumidity in the vicinity of user terminal 102 from a weather server 114.The weather information is available from the Internet and othersources. DTV location server 110 determines tropospheric propagationvelocity from the weather information using techniques such as thosedisclosed in B. Parkinson and J. Spilker, Jr. Global PositioningSystem-Theory and Applications, AIAA, Washington, D.C., 1996, Vol. 1,Chapter 17 Tropospheric Effects on GPS by J. Spilker, Jr.

DTV location server 110 can also receive from base station 104information which identifies a general geographic location of userterminal 102. For example, the information can identify a cell or cellsector within which a cellular telephone is located. This information isused for ambiguity resolution, as described below.

DTV location server 110 determines a position of the user terminal basedon the pseudo-ranges and a location of each of the transmitters (step206). FIG. 3 depicts the geometry of a position determination usingthree DTV transmitters 106. DTV transmitter 106A is located at position(x1, y1). The range between user terminal 102 and DTV transmitter 106Ais r1. DTV 106B transmitter is located at position (x2, y2). The rangebetween user terminal 102 and DTV transmitter 106B is r2. DTVtransmitter 106N is located at position (x3, y3). The range between userterminal 102 and DTV transmitter 106N is r3.

DTV location server 110 may adjust the value of each pseudo-rangeaccording to the tropospheric propagation velocity and the time offsetfor the corresponding DTV transmitter 106. DTV location server 110 usesthe phase center information from database 112 to determine the positionof each DTV transmitter 106.

User terminal 102 makes three or more pseudo-range measurements to solvefor three unknowns, namely the position (x, y) and clock offset T ofuser terminal 102. In other implementations, the techniques disclosedherein are used to determine position in three dimensions such aslongitude, latitude, and altitude, and can include factors such as thealtitude of the DTV transmitters.

The three pseudo-range measurements pr1, pr2 and pr3 are given bypr1=r1+T  (2)pr2=r2+T  (3)pr3=r3+T  (4)The three ranges can be expressed asr1=|X−X1|  (5)r2=|X−X2|  (6)r3=|X−X3|  (7)where X represents the two-dimensional vector position (x, y) of userterminal, X1 represents the two-dimensional vector position (x1, y1) ofDTV transmitter 106A, X2 represents the two-dimensional vector position(x2, y2) of DTV transmitter 106B, and X3 represents the two-dimensionalvector position (x3, y3) of DTV transmitter 106N. These relationshipsproduce three equations in which to solve for the three unknowns x, y,and T. DTV locations server 110 solves these equations according toconventional well-known methods. In an E911 application, the position ofuser terminal 102 is transmitted to E911 location server 116 fordistribution to the proper authorities. In another application, theposition is transmitted to user terminal 102.

In another implementation, user terminal 102 does not computepseudo-ranges, but rather takes measurements of the DTV signals that aresufficient to compute pseudo-range, and transmits these measurements toDTV location server 110. DTV location server 110 then computes thepseudo-ranges based on the measurements, and computes the position basedon the pseudo-ranges, as described above.

Position Location Performed by User Terminal

In another implementation, the position of user terminal 102 is computedby user terminal 102. In this implementation, all of the necessaryinformation is transmitted to user terminal 102. This information can betransmitted to user terminal by DTV location server 110, base station104, one or more DTV transmitters 106, or any combination thereof. Userterminal 102 then measures the pseudo-ranges and solves the simultaneousequations as described above. This implementation is now described.

User terminal 102 receives the time offset between the local clock ofeach DTV transmitter and a reference clock. User terminal 102 alsoreceives information describing the phase center of each DTV transmitter106 from a database 112.

User terminal 102 receives the tropospheric propagation velocitycomputed by DTV locations server 110. In another implementation, userterminal 102 receives weather information describing the airtemperature, atmospheric pressure, and humidity in the vicinity of userterminal 102 from a weather server 114, and determines troposphericpropagation velocity from the weather information using conventionaltechniques.

User terminal 102 can also receive from base station 104 informationwhich identifies the rough location of user terminal 102. For example,the information can identify a cell or cell sector within which acellular telephone is located. This information is used for ambiguityresolution, as described below.

User terminal 102 receives DTV signals from a plurality of DTVtransmitters 106 and determines a pseudo-range between the user terminal102 and each DTV transmitter 106. User terminal 102 then determines itsposition based on the pseudo-ranges and the phase centers of thetransmitters.

In any of these of the implementations, should only two DTV transmittersbe available, the position of user terminal 102 can be determined usingthe two DTV transmitters and the offset T computed during a previousposition determination. The values of T can be stored or maintainedaccording to conventional methods.

In one implementation, base station 104 determines the clock offset ofuser terminal 102. In this implementation, only two DTV transmitters arerequired for position determination. Base station 104 transmits theclock offset T to DTV location server 110, which then determines theposition of user terminal 102 from the pseudo-range computed for each ofthe DTV transmitters.

In another implementation, when only one or two DTV transmitters areavailable for position determination, GPS is used to augment theposition determination.

Receiver Architecture

FIG. 4 depicts an implementation 400 of a receiver for use in generatinga pseudo-range measurement. In one implementation, receiver 400 isimplemented within user terminal 102. In another implementation,receiver 400 is implemented within monitor units 108.

RF Sampler & Quantizer 406 sequentially tunes antenna 404 to each of thedigital TV signals 402 in the area, RF amplifies, and downconverts thesignal to IF or baseband. The wideband filtered signal with its 8 MHzbandwidth is then sampled and quantized by RF sampler and quantizer 406.Then a segment of the quantized signal including 4 or more symbolintervals is stored in memory 408. Preferably a substantially longersegment of perhaps 0.1 seconds or more in duration is used to improvethe averaging time and to improve noise performance.

Mixer 410 and correlator and integrator 412 sequentially correlate thestored time segment of the signal with various time offset versions ofthe reference scattered pilot carrier generated by scattered pilotgenerator 418. The reference signal is stepped in time by predeterminedtime steps to find the peak of the autocorrelation function. The stepsize is selected to produce a number of samples from the autocorrelationfunction that is sufficient to identify the autocorrelation peak. In oneimplementation, a large step size is initially used to obtain anestimate of the autocorrelation peak; then a smaller step size is usedto refine that estimate. As shown below, implementations of the presentinvention use time samples spaced by 1/(1116*20,000)=44 ns. A correlatorsearch control 420 searches for the major peak in the autocorrelationfunction and when found converts that measurement of pseudo-range todigitized form. Receiver 400 then sequentially performs the same set offunctions on the other digital TV signals 402 available in the area fromother DTV towers. It is not necessary to make multiple measurements fromsignals transmitted from the same DTV tower. The set of 3 or morepseudo-range measurements is then sent to DTV location server 110 by wayof digital cellular or other wireless link.

Note that the position location operation at the subscriber handset orother device need only take place when the subscriber needs positionlocation. For a subscriber walking slowly, in a slowly moving vehicle,or sitting in a building or field in an emergency, this locationinformation need only be measured infrequently. Thus the battery orother power source can be very small.

Although receiver 400 implements a cross-correlator with a sum ofdigital signals it will be clear to one skilled in the relevant artsthat alternate implementations can simplify the circuitry by usingFFT/DFT (fast Fourier transform/direct Fourier transform) processing forexample. Furthermore, although receiver 400 processes the samples atintermediate frequency (IF), other implementations process the samplesin analog or digital form, and can operate at IF or at baseband. Stillother implementations process the samples in the frequency domain.

Other signals within the DVB-T structure can also be used for positionlocation. For example, a wide laning technique could be applied to thecontinuous pilot signals. However, such techniques as wide laninginvolve inherent resolution of cycle ambiguities Techniques forresolving such ambiguities are well-known in the art. One such techniqueis disclosed in M. Rabinowitz, Ph.D. Thesis: A Differential CarrierPhase Navigation System Combining GPS with Low Earth Orbit Satellitesfor Rapid Resolution of Integer Cycle Ambiguities, 2000, Department ofElectrical Engineering, Stanford University, pages 59-76.

In receiver correlators and matched filters there are two importantsources of receiver degradation. The user terminal local oscillator isoften of relatively poor stability in frequency. This instabilityaffects two different receiver parameters. First, it causes a frequencyoffset in the receiver signal. Second, it causes the received bitpattern to slip relative to the symbol rate of the reference clock. Bothof these effects can limit the integration time of the receiver andhence the processing gain of the receiver. The integration time can beincreased by correcting the receiver reference clock. In oneimplementation a delay lock loop automatically corrects for the receiverclock.

In another implementation a NCO (numerically controlled oscillator)clock 414 adjusts the clock frequency of the receiver to match that ofthe incoming received signal clock frequency and compensate for driftsand frequency offsets of the local oscillator in user terminal 102.Increased accuracy of the clock frequency permits longer integrationtimes and better performance of the receiver correlator. The frequencycontrol input of NCO clock 414 can be derived from master clock 416, areceiver symbol clock rate synchronizer, tracking of the DVB-T pilotcarrier, or other clock rate discriminator techniques installed in NCOclock 414.

Position Location Enhancements

FIG. 5 illustrates a simple example of a position location calculationfor a user terminal 102 receiving DTV signals from two separate DTVantennas 106A and 106B. Circles of constant range 502A and 502B aredrawn about each of transmit antennas 106A and 106B, respectively. Theposition for a user terminal, including correction for the user terminalclock offset, is then at one of the intersections 504A and 504B of thetwo circles 502A and 502B. The ambiguity is resolved by noting that basestation 104 can determine in which sector 508 of its footprint (that is,its coverage area) 506 the user terminal is located. Of course if thereare more than two DTV transmitters in view, the ambiguity can beresolved by taking the intersection of three circles.

In one implementation, user terminal 102 can accept an input from a userthat gives a general indication of the area, such as the name of thenearest city. In one implementation, user terminal 102 scans availableDTV channels to assemble a fingerprint of the location. User terminal102 compares this fingerprint to a stored table that matches knownfingerprints with known locations to identify the current location ofuser terminal 102.

In one implementation the position location calculation includes theeffects of ground elevation. Thus in terrain with hills and valleysrelative to the phase center of the DTV antenna 106 the circles ofconstant range are distorted. FIG. 6 depicts the effects of a singlehill 604 on a circle of constant range 602 for a DTV transmitter 106that is located at the same altitude as the surrounding land.

The computations of user position are easily made by a simple computerhaving as its database a terrain topographic map which allows thecomputations to include the effect of user altitude on the surface ofthe earth, the geoid. This calculation has the effect of distorting thecircles of constant range as shown in FIG. 6.

DVB-T Signal Description

The current DVB-T signal is described in a document entitled DigitalVideo Broadcasting (DVB); Framing structure, channel coding andmodulation for digital terrestrial television, document number ETSI EN300 744, V1.4.1 (2001-01). The DVB-T signal is a complex orthogonalfrequency-division multiplexing (OFDM) signal that carries 188 Byte MPEG(Moving Picture Expert Group) packets using either 1512 or 6048 separatecarriers. Most of these components carry the random-like data modulationof the video TV signals and is less useful for precision tracking at lowsignal levels. Note that for our purposes of position location, the userterminal may be in locations where the entire information content of theDVB-T signal is not available.

However the DVB-T DTV signal has embedded in it additional componentsthat can be used through the techniques described herein for positionlocation. For example, the DVB-T DTV signal includes two types ofperiodic broadband pilot signals. The signal contains both a set ofcontinuous pilot carriers and a set of scattered pilot carriers. TheDVB-T signals have two modes: 2K and 8K. Some of the parameters of thesetwo modes are described in Table 1. While implementations of theinvention are described with reference to the 8K signals, the techniquesdescribed also apply to the 2K signals.

TABLE 1 Parameter 2K mode 8K mode Number of carriers K 1705 6817 SymbolDuration 224 microseconds 896 microseconds Carrier spacing 4464 Hz 1116Hz Total spacing of signal 7.61 MHz 7.61 MHzContinuous Pilot Signals

The DVB-T continuous pilot signals in the 8K mode are a set of 177carriers each having a constant reference binary ±1 amplitude selectedby a PN sequence described below. The carriers are spaced by 1116 Hz.The carrier numbers for the first 50 carrier frequencies are shown inFIG. 7. The frequency of a carrier can be found by taking the product ofthe carrier number and 1116 Hz. FIG. 8 depicts the first 50 carriers ofthe continuous pilot carriers. The vertical scale is the carrier number.The minimum frequency offset between any two continuous pilot carriersis 3×1116 Hz which determines the time ambiguity of these continuouscarriers. This signal can be likened to a sidetone ranging signalcommonly used in ranging measurements. However it differs in that thepower is divided among 177 separate carriers. Instead however one cancorrelate the signal with a composite reference waveform of 177 carriersgenerated in FFT fashion. However this composite continuous pilot signalhas a poor autocorrelation function with many significant spectralsidelobes as shown in FIG. 9.

FIG. 9 depicts the autocorrelation function of the composite continuouspilot carriers with 177 parallel carriers in the 8K mode. The timeincrements are given on the horizontal scale in increments of 1/1116 s.The signal was sampled at a rate of 1116×20,000 samples/s. However ascan be seen, the sidelobe levels of this signal are quite high with manypeaks above 0.2 in magnitude.

Scattered Pilot Carriers

The 8K scattered pilot carriers are a set of 568 uniformly-spaced pilotcarriers, each frequency hopped in a chirp-like fashion over 4sequentially increasing frequencies. Thus each pilot begins at afrequency that is a multiple of 12×1116 Hz and remains at that frequencyfor the remainder of a symbol duration (1/1116 s). Then for the nextsymbol the pilot hops to a new frequency that is higher by 3×1116 Hz andhas a new ±1 sign. The pilot repeats this increase for a total of 3increments and then returns to its original frequency. The frequencyhopping of the first 5 of these 568 scattered pilots is shown in FIG.10. In each time increment the pilot carrier increases in frequency by 3increments of 1116 Hz. Each of the 6816/12=568 scattered pilots isspaced by 12×1116=13,392 Hz. For the 2K mode there are 142 scatteredpilots spaced by 53,568 Hz.

Each pilot carrier is given a ±1 sign amplitude as governed by a PNsequence of an 11 stage shift register with a polynomialx¹¹+x₂+1  (8)

This PN sequence generates a sequencew[k]=±1  (9)where k is the frequency of the individual pilot carrier as given above.Thus each time a pilot carrier changes to a new frequency it alsochanges its sign according to w[k].

Thus the frequency of each of the scattered pilots can be expressed interms of t and p ask[t,p]=3 Mod[n[t],4]+12p  (10)where p is the number of the pilot and n[t] is the quantized timeintervaln[t]=└1116t┘  (11)

Each signal component for each of the 568 values of p iss[t,p]=w[k[t,p]sin[2πk[t,p]×1116t]]  (12)

The total scattered pilot signal is then the sum of 568 frequency hoppedindividual pilot carriers

$\begin{matrix}{{s_{total}\lbrack t\rbrack} = {\sum\limits_{p = 0}^{p = 862}{s\left\lbrack {t,p} \right\rbrack}}} & (13)\end{matrix}$

FIG. 11 depicts the waveform of one example carrier with no signreversals over 8 time increments. Time is given in seconds. Thisscattered pilot carrier then has a total of 6816/12=568 carriers each ofwhich hops sequentially over 4 frequencies for a total of 568×4=2272total frequencies in a chirp-like fashion.

FIG. 12 is another view of the scattered pilot carriers. In FIG. 12, thediagonal lines represent the 568 8K scattered pilot carriers chirpingstepwise over the entire band of 7.61 MHz (6816 carrier numbers) in6816/4=1704 symbol intervals. Thus at any one time instant there are 568simultaneous chirp carriers. Each chirp carrier sweeps the entire 7.61MHz frequency band in stepwise fashion. The numbers shown are for the 8Kmode where the symbol duration is 896 microseconds (us). Thecorresponding numbers for the 2K mode are shown in parentheses where thesymbol duration is 224 us.

This signal has a very good autocorrelation function as shown in FIGS.13-15. FIG. 13 depicts the autocorrelation function of the composite setof 568 frequency-hopped scattered pilot carriers. The composite signalhas been sampled at a rate 1116×20,000=22.32 MHz. Thus there are 80,000samples over the 4-symbol time increment period of the scattered pilotcarriers. Note the very low sidelobe cross-correlation of this signalwith the exception of the 4 sidelobes which as shown below are doublets.FIGS. 14 and 15 show the detail over much smaller time increments.

FIG. 14 shows the detailed fine structure of the scattered pilotcomposite signal observed over the first 100 time increments. Note thelow levels of the autocorrelation function outside of the peak.

FIG. 15 shows the fine structure of the doublet sidelobe of thescattered pilot composite carrier. Note again the very small values ofthe autocorrelation function of this signal outside of the main peak andthe 4 sidelobe peaks.

Monitor Units

FIG. 16 depicts an implementation 1600 of monitor unit 108. An antenna1604 receives GPS signals 1602. A GPS time transfer unit 1606 develops amaster clock signal based on the GPS signals. In order to determine theoffset of the DTV transmitter clocks, a NCO (numerically controlledoscillator) code synchronization timer 1608A develops a mastersynchronization signal based on the master clock signal. The mastersynchronization signal can include the DVB-T scattered pilot carriers.In one implementation, the NCO field synchronization timers 1608A in allof the monitor units 108 are synchronized to a base date and time. Inimplementations where a single monitor unit 108 receives DTV signalsfrom all of the same DTV transmitters that user terminal 102 does, it isnot necessary to synchronize that monitor unit 108 with any othermonitor unit for the purposes of determining the position of userterminal 102. Such synchronization is also unnecessary if all of themonitor stations 108, or all of the DTV transmitters, are synchronizedto a common clock.

A DTV antenna 1612 receives a plurality of DTV signals 1610. In anotherimplementation, multiple DTV antennas are used. An amplifier 1614amplifies the DTV signals. One or more DTV tuners 1616A through 1616Neach tunes to a DTV channel in the received DTV signals to produce a DTVchannel signal. Each of a plurality of NCO code synchronization timers1608B through 1608M receives one of the DTV channel signals. Each of NCOcode synchronization timers 1608B through 1608M extracts a channelsynchronization signal from a DTV channel signal. The channelsynchronization signal can include the DVB-T scattered pilot carriers.In one implementation, the continuous pilot signal and symbol clocksignal within the DVB-T signal are used as acquisition aids.

Each of a plurality of summers 1618A through 1618N generates a clockoffset between the master synchronization signal and one of the channelsynchronization signals. Processor 1620 formats and sends the resultingdata to DTV location server 110. In one implementation, this dataincludes, for each DTV channel measured, the identification number ofthe DTV transmitter, the DTV channel number, the antenna phase centerfor the DTV transmitter, and the clock offset. This data can betransmitted by any of a number of methods including air link and theInternet. In one implementation, the data is broadcast in spare MPEGpackets on the DTV channel itself.

Software Receivers

One thorough approach to mitigating the effects of multipath is tosample an entire autocorrelation function, rather than to use only earlyand late samples as in a hardware setup. Multipath effects can bemitigated by selecting the earliest correlation peak.

In the case that position can be computed with a brief delay, such as inE911 applications, a simple approach is to use a software receiver,which samples a sequence of the filtered signal, and then processes thesample in firmware on a DSP.

FIG. 17 illustrates one implementation 1700 for tracking in software. Anantenna 1702 receives a DTV signal. Antenna 1702 can be a magneticdipole or any other type of antenna capable of receiving DTV signals. Abandpass filter 1704 passes the entire DTV signal spectrum to an LNA1706. In one implementation, filter 1704 is a tunable bandpass filterthat passes the spectrum for a particular DTV channel under the controlof a digital signal processor (DSP) 1714.

A low-noise amplifier (LNA) 1706 amplifies and passes the selectedsignal to a DTV channel selector 1708. DTV channel selector 1708 selectsa particular DTV channel under the control of DSP 1714, and filters anddownconverts the selected channel signal from UHF (ultra-high frequency)to IF (intermediate frequency) according to conventional methods. Anamplifier (AMP) 1710 amplifies the selected IF channel signal. Ananalog-to-digital converter and sampler (A/D) 1712 produces digitalsamples of the DTV channel signal s(t) and passes these samples to DSP1714.

Now the processing of the DTV channel signal by DSP 1714 is describedfor a coherent software receiver. A nominal offset frequency for thedownconverted sampled signal is assumed. If this signal is downconvertedto baseband, the nominal offset is 0 Hz. The process generates thecomplete autocorrelation function based on samples of a signal s(t). Theprocess may be implemented far more efficiently for a low duty factorsignal. Let T_(i) be the period of data sampled, ω_(in) be the nominaloffset of the sampled incident signal, and let ω_(offset) be the largestpossible offset frequency, due to Doppler shift and oscillator frequencydrift. The process implements the pseudocode listed below.

-   -   R_(max)=0    -   Create a complex code signal        s _(code)(t)=C _(i)(t)+jC _(q)(t)

where C_(i) is the function describing the in-phase baseband signal andC_(q) is the function describing the quadrature baseband signal.

-   -   Compute F{s_(code)}^(※) where F is the Fourier transform        operator, and ※ is the conjugate operator.    -   For ω=ω_(in)−ω_(offset) to ω_(in)+ω_(offset) step

$\frac{\pi}{2T_{i}}$

-   -   -   Create a complex mixing signal            s _(mix)(t)=cos(ωt)+j sin(ωt), t=[0 . . . T_(i)]        -   Combine the incident signal s(t) and the mixing signal            s_(mix)(t)            s _(comb)(t)=s(t)s _(mix)(t)        -   Compute the correlation function            R(τ)=F⁻¹{F(s_(code))F(s_(comb))}        -   If max_(τ)|R(τ)|>R_(max), R_(max)←max_(τ)|R(τ)|,            R_(store)(τ)=R(τ)

    -   Next ω

Upon exit from the process, R_(store)(τ) will store the correlationbetween the incident signal s(t) and the complex code signals_(code)(t). R_(store)(τ) may be further refined by searching oversmaller steps of ω. The initial step size for ω must be less then halfthe Nyquist rate

$\frac{2\pi}{T_{i}}.$The time offset τ that produces the maximum correlation output is usedas the pseudo-range.

Alternative Embodiments

The invention can be implemented in digital electronic circuitry, or incomputer hardware, firmware, software, or in combinations thereof.Apparatus of the invention can be implemented in a computer programproduct tangibly embodied in a machine-readable storage device forexecution by a programmable processor; and method steps of the inventioncan be performed by a programmable processor executing a program ofinstructions to perform functions of the invention by operating on inputdata and generating output. The invention can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. Each computer program can be implemented ina high-level procedural or object-oriented programming language, or inassembly or machine language if desired; and in any case, the languagecan be a compiled or interpreted language. Suitable processors include,by way of example, both general and special purpose microprocessors.Generally, a processor will receive instructions and data from aread-only memory and/or a random access memory. Generally, a computerwill include one or more mass storage devices for storing data files;such devices include magnetic disks, such as internal hard disks andremovable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM disks. Any of the foregoing canbe supplemented by, or incorporated in, ASICs (application-specificintegrated circuits).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

For example, while various signals and signal processing techniques arediscussed herein in analog form, digital implementations will beapparent to one skilled in the relevant art after reading thisdescription.

For example, although one method for tracking the DVB-T signal isdescribed, it should be clear that there are several methods of trackingthese signals using various forms of conventional delay lock loops andthrough the use of various types of matched filters.

While implementations of the invention are discussed with reference tothe 8 MHz signal, implementations can be used with signals of otherbandwidths. Further, implementations of the invention can employ asubset of the bandwidth of the DVB-T signal. For example, animplementation of the invention can achieve satisfactory results usingonly 6 MHz of an 8 MHz DVB-T signal. Implementations of the inventioncan be extended to use future enhancements to the DVB-T signal.

Implementations of the present invention exploit the low duty factor ofthe DTV signal in many ways. For example, one implementation employs atime-gated delay-lock loop (DLL) such as that disclosed in J. J.Spilker, Jr., Digital Communications by Satellite, Prentice-Hall,Englewood Cliffs N.J., 1977, Chapter 18-6. Other implementations employvariations of the DLL, including coherent, noncoherent, andquasi-coherent DLLs, such as those disclosed in J. J. Spilker, Jr.,Digital Communications by Satellite, Prentice-Hall, Englewood CliffsN.J., 1977, Chapter 18 and B. Parkinson and J. Spilker, Jr., GlobalPositioning System-Theory and Applications, AIAA, Washington, D.C.,1996, Vol. 1, Chapter 17, Fundamentals of Signal Tracking Theory by J.Spilker, Jr. Other implementations employ various types of matchedfilters, such as a recirculating matched filter.

In some implementations, DTV location server 110 employs redundantsignals available at the system level, such as pseudo-ranges availablefrom the DTV transmitters, making additional checks to validate each DTVchannel and pseudo-range, and to identify DTV channels that areerroneous. One such technique is conventional receiver autonomousintegrity monitoring (RAIM).

Accordingly, other embodiments are within the scope of the followingclaims.

1. An apparatus to determine the position of a user terminal,comprising: a receiver to receive, at the user terminal, a wirelessorthogonal frequency-division multiplexing (OFDM) signal comprising ascattered pilot signal; and a processor to determine a pseudo-rangebased on the scattered pilot signal; wherein a position of the userterminal is determined based on the pseudo-range and a location of atransmitter of the OFDM signal.
 2. The apparatus of claim 1, wherein theOFDM signal comprises: a Digital Video Broadcasting (DVB) signal.
 3. Theapparatus of claim 2, wherein the DVB signal comprises at least one of:a DVB-Terrestrial (DVB-T) signal; and a DVB-Handheld (DVB-H) signal. 4.The apparatus of claim 1: wherein the processor determines the positionof the user terminal based on the pseudo-range and the location of thetransmitter of the OFDM signal.
 5. The apparatus of claim 4: wherein, todetermine the position of the user terminal, the processor determines anoffset between a local time reference in the user terminal and a mastertime reference, and determines the position of the user terminal basedon the wireless OFDM signal, the location of the transmitter of the OFDMsignal, and the offset.
 6. The apparatus of claim 5: wherein theprocessor determines a subsequent position of the user terminal usingthe offset.
 7. An apparatus to determine the position of a userterminal, comprising: receiver means for receiving, at the userterminal, a wireless orthogonal frequency-division multiplexing (OFDM)signal comprising a scattered pilot signal; and processor means fordetermining a pseudo-range based on the scattered pilot signal; whereina position of the user terminal is determined based on the pseudo-rangeand a location of a transmitter of the OFDM signal.
 8. The apparatus ofclaim 7, wherein the OFDM signal comprises: a Digital Video Broadcasting(DVB) signal.
 9. The apparatus of claim 8, wherein the DVB signalcomprises at least one of: a DVB-Terrestrial (DVB-T) signal; and aDVB-Handheld (DVB-H) signal.
 10. The apparatus of claim 7: wherein theprocessor means determines the position of the user terminal based onthe pseudo-range and the location of the transmitter of the OFDM signal.11. The apparatus of claim 10: wherein, to determine the position of theuser terminal, the processor means determines an offset between a localtime reference in the user terminal and a master time reference, anddetermines the position of the user terminal based on the wireless OFDMsignal, the location of the transmitter of the OFDM signal, and theoffset.
 12. The apparatus of claim 11: wherein the processor meansdetermines a subsequent position of the user terminal using the offset.13. A method for determining the position of a user terminal,comprising: receiving, at the user terminal, a wireless orthogonalfrequency-division multiplexing (OFDM) signal comprising a scatteredpilot signal; and determining a pseudo-range based on the scatteredpilot signal; wherein a position of the user terminal is determinedbased on the pseudo-range and a location of a transmitter of the OFDMsignal.
 14. The method of claim 13, wherein the OFDM signal comprises: aDigital Video Broadcasting (DVB) signal.
 15. The method of claim 14,wherein the DVB signal comprises at least one of: a DVB-Terrestrial(DVB-T) signal; and a DVB-Handheld (DVB-H) signal.
 16. The method ofclaim 13, further comprising: determining an offset between a local timereference in the user terminal and a master time reference; anddetermining the position of the user terminal based on the wireless OFDMsignal, the location of the transmitter of the OFDM signal, and theoffset.
 17. The method of claim 16, further comprising: determining asubsequent position of the user terminal using the offset. 18.Computer-readable media embodying instructions executable by a computerto perform a method for determining the position of a user terminal, themethod comprising: receiving, at the user terminal, a wirelessorthogonal frequency-division multiplexing (OFDM) signal comprising ascattered pilot signal; and determining a pseudo-range based on thescattered pilot signal; wherein a position of the user terminal isdetermined based on the pseudo-range and a location of a transmitter ofthe OFDM signal.
 19. The computer-readable media of claim 18, whereinthe OFDM signal comprises: a Digital Video Broadcasting (DVB) signal.20. The computer-readable media of claim 19, wherein the DVB signalcomprises at least one of: a DVB-Terrestrial (DVB-T) signal; and aDVB-Handheld (DVB-H) signal.
 21. The computer-readable media of claim18, wherein the method further comprises: determining an offset betweena local time reference in the user terminal and a master time reference;and determining the position of the user terminal based on the wirelessOFDM signal, the location of the transmitter of the OFDM signal, and theoffset.
 22. The computer-readable media of claim 21, wherein the methodfurther comprises: determining a subsequent position of the userterminal using the offset.